Both frequency and phase angle are critical measurements in an electric power grid. Accurate measurements of frequency and phase angle are now accessible primarily due to the availability of an accurate world wide time synchronization signal via the GPS constellation. The timing synchronization signals are accurate to better than one microsecond and are reported in UTC time.
Using these new sensors, one can now measure frequency to better than one part in 10,000 and absolute phase angle to better than one part in 1,000.
The US Electric power grid comprises a number of “control areas or balancing authorities,” each responsible for maintaining a balance between its generation and load. If persistent under-frequency occurs, at least one control area is consuming more power from the grid than it is supplying. This lowers frequency and shifts the phase angles. Minor deviations at the control area level are routine. The North American Electric Reliability Council (NERC) has established operating rules that specify maximum permissible deviations, and focus on prohibiting persistent deviations, but not instantaneous ones. Each control area computes a measure of performance called the Area Control Error (ACE). This value should be zero for stable operation of the grid. This measure determines how well each control area matches its load and generation.
It has been common practice to estimate the phase angles using a network model with normal measures of generation and load. This involves a traditional “load flow” solution in combination with measurements. This process is called “State Estimation”. The solution to this problem involves solving a large dimensional problem, typically up to 30,000 nodes. A solution is not always guaranteed and it is highly dependent on the real time topology of the grid. This is not always known with certainty.